The present invention relates to an image forming apparatus such as a digital photocopier in which the photosensitive drum is scanned by a plurality of laser beams at the same time while copying, and thus a static latent image is formed on the photosensitive drum.
Recently, various types of digital photocopiers that form images by scanning exposure surface with a laser beam and by the electrophotographic process have been developed. More recently, in order to increase the speed of image formation, a multi-beam type digital photocopier has been developed, in which a plurality of laser beams are generated and a plurality of lines are simultaneously scanned at each time with these laser beams.
Such a multi-mean type digital photocopier includes a plurality of semiconductor laser oscillators that generate laser beams, a galvano-mirror that controls a sub-scanning directional position of each laser beam output from those laser oscillators, a polygon mirror that directs each laser beam reflecting on the galvano-mirror to further reflect it towards the photographic drum, thereby scanning the photosensitive drum with each laser beam, and a lens-system unit mainly consisting of a collimator lens and an f-xcex8 lens.
In order to form an image at an accurate location on a sheet, the exposure positions in both of the main scanning direction and sub-scanning direction must be accurately adjusted, especially in the case of the multi-beam type apparatus. Usually, the adjustment of the beam scanning position is carried out at the time of a so-called warming-up state in which the power is turned on and each part of the apparatus is initialized, a stand-by state in which the warming-up is completed and the apparatus is ready to copy, and a state immediately before copying, that is, when the copy start button is pushed.
In the multi-beam type digital photocopier described above, the beam position in the sub-scanning direction and the beam position in the main scanning direction are controlled with regard to each laser beam, and thus the positional error of each beam is adjusted within several xcexcm. The adjustment of the beam position in the sub-scanning direction is carried out by giving an instruction value to the galvano-mirror until the error becomes an allowance value or less. The adjustment of the beam exposure position in the main scanning direction is carried out by a pixel clock generation circuit and a delay circuit serving to delay the pixel clock by the unit of a fraction of an exposure time for one pixel. In general, it requires a relatively long processing time to adjust the beam scanning position in multi-beam type digital photocopiers, and therefore there is a demand for shortening this adjustment processing time.
Under the above-described circumstances, the object of the present invention is to shorten the time required to adjust the beam scanning position and thereby to improve the overall copying processing capability of the apparatus.
In order to achieve the above object, according to one aspect of the present invention, there is provided a light beam scanning apparatus comprising:
a beam generating unit configured to generate a light beam;
a scanning unit configured to reflect the light beam generated from the beam generating unit to a surface to be scanned, and scanning the surface to be scanned with the light beam in a main scanning direction;
a beam passing position detecting unit configured to detect a passing position of the light beam in a sub-scanning direction, which is scanned by the scanning unit and provide a position signal that indicates the passing position;
a correcting unit configured to correct the position signal from the passing position detecting unit and provide corrected position signal; and
an adjusting unit configured to adjust a correction amount of the correcting unit such that the corrected position signal falls within a target range, the target range being variable in accordance with the position of the light beam.